The spin-resolved electronic states of CO molecules adsorbed on Fe(110) surfaces are investigated using spin-polarized metastable-atom deexcitation spectroscopy (SPMDS) measurements and first-principles calculation. The existence of the adsorbate-induced 2 pi(*) state, which is partially filled by electron backdonation, is detected directly by SPMDS and reproduced by the calculation of local density of states and subtracted densities. Positive spin asymmetries for 4 sigma, 5 sigma/1 pi, and 2 pi(*) peaks observed in SPMDS spectra are well described by the calculated spin density and plane-averaged density of states, which indicate negative-spin polarization towards vacuum side even at the low CO coverage.